1. Field of the Invention
The present invention relates to data communications, and more particularly, to channel shortening techniques for data communications.
2. Description of the Related Art
Bi-directional digital data transmission systems are presently being developed for high-speed data communication. One standard for high-speed data communications over twisted-pair phone lines that has developed is known as Asymmetric Digital Subscriber Lines (ADSL). Another standard for high-speed data communications over twisted-pair phone lines that is presently proposed is known as Very High Speed Digital Subscriber Lines (VDSL).
The Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has finalized a discrete multi tone based approach for the transmission of digital data over ADSL. The standard is intended primarily for transmitting video data and fast Internet access over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard (hereinafter ADSL standard). Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to 8 million bits per second (Mbits/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 xe2x80x9ctonesxe2x80x9d or xe2x80x9csub-channelsxe2x80x9d that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when modulation is performed efficiently using an inverse fast Fourier transform (IFFT), typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.
The ADSL standard also defines the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term ADSL comes from the fact that the data transmission rate is substantially higher in the downstream direction than in the upstream direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over telephone lines.
Because both downstream and upstream signals travel on the same pair of wires (that is, they are duplexed) they must be separated from each other in some way. The method of duplexing used in the ADSL standard is Frequency Division Duplexing (FDD) or echo canceling. In frequency division duplexed systems, the upstream and downstream signals occupy different frequency bands and are separated at the transmitters and receivers by filters. In echo cancelled systems, the upstream and downstream signals occupy the same frequency bands and are separated by signal processing.
ANSI is producing another standard for subscriber line based transmission system, which is referred to as the VDSL standard. The VDSL standard is intended to facilitate transmission rates of at least 12.98 Mbit/s and up to 51.92 Mbit/s or greater in the downstream direction. To achieve these rates, the transmission distance over twisted-pair phone lines must generally be shorter than the lengths permitted using ADSL. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the xe2x80x9ccurbxe2x80x9d to the customer premise is standard unshielded twisted-pair (UTP) telephone lines.
A number of modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). Most of the proposed VDSL/FTTC modulation schemes utilize frequency division duplexing of the upstream and downstream signals. Another promising proposed VDSL/FTTC modulation scheme uses periodic synchronized upstream and downstream communication periods that do not overlap with one another. That is, the upstream and downstream communication periods for all of the wires that share a binder are synchronized. With this arrangement, all the very high speed transmissions within the same binder are synchronized and time division duplexed such that downstream communications are not transmitted at times that overlap with the transmission of upstream communications. This is also referred to as a (i.e. xe2x80x9cping pongxe2x80x9d) based data transmission scheme. Quiet periods, during which no data is transmitted in either direction, separate the upstream and downstream communication periods. For example, with a 20-symbol superframe, two of the DMT symbols in the superframe are silent (i.e., quite period) for the purpose of facilitating the reversal of transmission direction on the phone line. In such a case, reversals in transmission direction will occur at a rate of about 4000 per second. For example, quiet periods of about 10-25 xcexcs have been proposed. The synchronized approach can be used a wide variety of modulation schemes, including multi-carrier transmission schemes such as Discrete Multi-Tone modulation (DMT) or Discrete Wavelet Multi-Tone modulation (DWMT), as well as single carrier transmission schemes such as Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase Shift Keying (QPSK), or vestigial sideband modulation. When the synchronized time division duplexed approach is used with DMT it is referred to as synchronized DMT (SDMT).
Multicarrier modulation has been receiving a large amount of attention due to the high data transmission rates it offers. FIG. 1A is a basic block diagram of a conventional multicarrier transmitter 10. The multicarrier transmitter 10 receives serial input data at a rate Mfs bit/s. The serial input data is grouped by a serial-to-parallel converter 12 into blocks of M bits at a symbol rate of fs. The M bits are used by modulators 14 to modulate Nc carriers (mn bits for carrier n) which are spaced xcex94fc apart across a usable frequency band. The modulated signals are then summed by an adder 16 and transmitted. In a receiver, the received signal is demodulated by each of the Nc carriers, and mn bits are recovered from each carrier. A more detailed discussion of the principals of multicarrier transmission and reception is provided in J. A. C. Bingham, xe2x80x9cMulticarrier Modulation For Data Transmission: An Idea Whose Time Has Come,xe2x80x9d IEEE Communications Mag., pp. 5-14, May 1990.
FIG. 1B is a block diagram of a conventional multicarrier modulation system 100. The multicarrier modulation system 100 is generally known in the art and discussed in, for example, U.S. Pat. No. 5,285,474, which is hereby incorporated by reference. The multicarrier modulation system 100 has a transmitter side and a receiver side. The transmitter side includes an encoder 102 that receives digital signals to be transmitted. The encoder 102 encodes the digital signals and then passes the encoded signals to a IFFT unit 104 that modulates the encoded signals on multiple carriers. The modulated signals are then converted to analog signals by a digital-to-analog converter 106. The resulting analog signals are then transmitted to a receiver over a channel 108.
The receiver side of the multicarrier modulation system 100 operates to receive the transmitted analog signals from the transmitter side through the channel 108. The received analog signals are converted into digital signals by an analog-to-digital converter 110. The digital signals are then supplied to a time-domain equalizer (TEQ) 112 that compensates for the attenuation and delay on each of the subchannels. The resulting signals are then supplied to a FFT unit 114 that converts the resulting signals from the time domain to the frequency domain. The frequency domain signals are then supplied to a frequency-domain equalizer (FEQ) 116 that compensates for the attenuation and delay of each of the subchannels. In effect, the FFT unit 114 and the FEQ 116 operate to demodulate the digital signals from the multiple carriers. The signals produced by the FEQ 116 are then supplied to a decoder 118 to recover the data signals originally transmitted.
Distortion by the channel 108 causes amplitude and delay variations in the channel responses. These amplitude and delay variations can lead to errors in the recovery of the transmitted data at the receiver. The time domain equalizer (TEQ) 112 provided on the receiver side of the multicarrier modulation system 100 operates to attempt to equalize the amplitude and delay of the channel responses over the frequency band. The operation of the TEQ 112 is computationally complex and not able to perfectly equalize the amplitude and delay of the channel response. Given the computational difficulty in performing time domain equalization, the time domain equalization is typically performed by an adaptive equalization technique such as discussed in U.S. Pat. No. 5,285,474. Adaptive equalization has been somewhat effective but occasionally has difficulty obtaining convergence.
With multicarrier modulation systems that provide upstream and downstream data transfer, a guard period (e.g., cyclic prefix) is typically added to each symbol to increase the length of the sample. In this case, the time domain equalization operates to compress the channel response to the length of the guard period using some sort of adaptive equalization. This channel compression is known as channel shortening. By providing the guard period and utilizing the time domain equalization, the distorted transient response of a channel is largely mitigated and intersymbol and intercarrier interference is substantially reduced. The combination of the guard band and adaptive equalization is needed when the channel response (length) is long. However, the convergence of the adaptive equalization is not fast, accurate or guaranteed when the length of the guard band is small compared to the length of the channel.
Although conventional time domain equalization techniques have reduced channel distortion by shortening the effective length of the channel, in many cases the effective lengths are still too long and too much channel distortion remains present. Although providing a guard band is helpful, the length of the guard band needs to remain relatively small so as to not hinder the efficiency of data transmissions, and thus, time domain equalization techniques are heavily relied on to reduce channel distortion.
Hence, there is a need for more effective techniques to shorten effective channel lengths, particularly in multicarrier modulation systems where distortion is particularly problematic.
Broadly speaking, the invention relates to time domain equalization. More particularly, the invention relates to the following aspects (i) improved time domain equalization techniques referred to as poly-path time domain equalization techniques; (ii) improved training methods for training transmitters and/or receivers of a data transmission system; and (iii) a data transmission system in which a transmitter side provides time domain equalization. These aspects are particularly suitable for time domain equalization in multicarrier modulation systems where channel shortening provided by time domain equalization is particularly needed.
The invention can be implemented in numerous ways, including as an apparatus, system, method, or computer readable media. Several embodiments of the invention are discussed below.
As an apparatus for recovering data transmitted by a transmitter of a DMT transmission system, one embodiment of the invention includes: an analog-to-digital converter to receive transmitted analog signals and produce digital signals therefrom, the transmitted analog signals being time domain signals representing data transmitted; a poly-path time domain equalizer to provide a plurality of paths for the received digital signals and perform time domain equalization on the received digital signals on both of the paths to produce equalized digital signals; a multicarrier demodulator to receive the equalized digital signals and demodulate the equalized digital signals to produce digital frequency domain data; and a decoder to decode the digital frequency domain data to recover the data transmitted.
As a method for shortening an effective channel length of an actual channel in a DMT data transmission system, an embodiment of the invention includes the operations of: receiving analog signals transmitted over the actual channel using DMT modulation; converting the received analog signals to digital signals; forming a plurality of effective channels from the received analog signals; providing a FIR filter for each of the effective channels; determining filter taps for each of the FIR filters such that an overall effective channel length is shortened; and shortening the overall effective channel length using the FIR filters associated therewith and using a different set of the determined filter taps for each of the FIR filters.
As a method for shortening an effective channel length of an actual channel prior to data transmission over the actual channel in a multicarrier-based data transmission system, an embodiment of the invention includes the operations of: receiving digital signals to be transmitted over the actual channel; forming a plurality of effective channels from the received digital signals; providing a FIR filter for each of the effective channels; determining filter taps for each of the FIR filters such that an overall effective channel length is shortened; and shortening the overall effective channel length using the FIR filters associated therewith and using a different set of the determined filter taps for each of the FIR filters.
As remote receiver apparatus for a data transmission system, an embodiment of the invention includes a transmitter side, a receiver side, and a hybrid circuit operatively connecting the transmitter side and the receiver side to a channel. The transmitter side includes at least an encoder, a multicarrier modulator, a time domain equalizer, and a digital-to-analog converter. The encoder operates to encode data to be transmitted from the remote receiver apparatus. The multicarrier modulator operates to modulate the encoded data to produce modulated signals. The time domain equalizer operates to performs time domain equalization on the modulated signals to produce equalized digital signals. The digital-to-analog converter receives the equalized digital signals and produces analog signals to be transmitted therefrom. The receiver side including at least an analog-to-digital converter, a time domain equalizer, a multicarrier demodulator, and a decoder. The analog-to-digital converter receives analog signals associated with data that has been transmitted and produces digital signals therefrom. The time domain equalizer performs time domain equalization on the received digital signals to produce equalized digital signals. The multicarrier demodulator receives the equalized digital signals and demodulates the equalized digital signals to produce demodulated data. The decoder operates to decode the demodulated data to recover the data transmitted.
As a transmitter for a multicarrier-based data transmission system, an embodiment of the invention includes: an encoder to encode data to be transmitted by the transmitter; a multicarrier modulator to modulate the encoded data to produce modulated signals; a time domain equalizer to performs time domain equalization on the modulated signals to produce equalized digital signals; and a digital-to-analog converter to receive the equalized digital signals and produce analog signals to be transmitted therefrom.
As computer readable media, embodiments of the invention would include computer readable program code for performing the operations of the methods according to the invention.
The various aspects of invention have numerous advantageous. One advantage of the invention is improved compensation for channel distortion (or improved channel shortening), which means a better signal-to-noise ratio for the data transmissions. Another advantage is more accurate determinations of filter taps utilized in Finite Impulse Response (FIR) filters. Still another advantage of the invention is the ability to train a chosen target channel. Yet another advantage is more accurate modeling of channels. Still yet another advantage is the ability to better manage power consumption at a central transmission site.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.